I have found this article that states and I quote:
Motorola updated their Touchless Control app this evening to include a
feature that allows you to ask your phone “What’s up?” in order to
hear what your current notification situation is like. By asking “Ok,
Google Now, what’s up?” your phone will read the time followed by
notifications back to you, letting you decide if any are important
enough to act upon.
What is Touchless Control though? According to this page:
Touchless Control is a separate application from Google Voice Search
but it works with Google Voice Search. Touchless Control's main
purpose is to wait for the launch phrase, "OK Google" or "Ok Google
(Now)" and then either execute the command or send the request to
Google Voice Search.
In the same page you'll see that What's up is a command for Touchless Control that reads notifications. Touchless Control is now called Moto Voice.
Is that a feature specific to the Moto X (or, at least, to Motorola
devices)?
Short answer is yes. The reason same command doesn't work now, is because the original feature was originating from a for-Motorola-only app.
How can I get Google Assistant to read back my phone notifications?
Even though the app was made for Motorola phones and has official support only for them, there are ways to get it on other phones too according to this page.